1. Field of the Invention
The present invention relates to a method for producing 3,4-dihydro-1,2,3-oxathiazin-4-one-2,2-dioxide compounds or salts thereof which are useful typically as sweeteners or raw materials therefor in food industry or intermediate materials for fine chemicals.
2. Description of the Related Art
A method for producing a 3,4-dihydro-1,2,3-oxathiazin-4-one-2,2-dioxide compound or a salt thereof is disclosed, for example, in Angewandte Chemie International Edition, 12, 10 (1973), 869-876 (or German Unexamined Patent Application Publication No. 2453063). In this method, an acetoacetamide-N-sulfonyl halide (fluoride or chloride) is cyclized by the action of a base such as methanolic KOH to thereby yield a 3,4-dihydro-1,2,3-oxathiazine, such as 6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one-2,2-dioxide or a salt thereof. The acetoacetamide-N-sulfonyl halide is prepared by allowing a halosulfonyl isocyanate to react with a suitable acetoacetylating agent. Such a method, however, is not suitable as an industrial method, since the halosulfonyl isocyanate and amidosulfonyl halide used as starting materials are special compounds and are difficult to handle.
Separately, Japanese Unexamined Patent Application Publication No. 03-184948, Japanese Examined Patent Application Publications No. 03-54940, No. 05-70627 and No. 06-25189 each disclose a method for producing 6-methyl-3,4-dihydro-1,2,3-oxathiazin-4-one-2,2-dioxide or a salt thereof by allowing acetoacetamide-N-sulfonic acid or a salt thereof to react with SO3 in an inert organic solvent, subjecting the product to cyclization and then to ring-closing. Among them, Japanese Examined Patent Application Publication No. 03-54940 discloses a spray tower reactor and a thin-film reactor equipped with a mechanical stirrer as an apparatus for carrying out the cyclization at low temperatures within a short time and for achieving a high yield. These apparatuses employ not only external cooling but also heat removal by the action of latent heat of vaporization of a solvent, in order to remove a large quantity of heat generated in the cyclization and dehydration. When a spray tower or a thin-film evaporator equipped with a mechanical stirrer, such as a wiped film evaporator (WFE), is used as the reactor in the production of a 3,4-dihydro-1,2,3-oxathiazin-4-one-2,2-dioxide compound or a salt thereof, the apparatus (reactor) is expensive and exhibits insufficient stability in production. This is because the residence time and reaction control in the reactor significantly affects the reaction result but the apparatus is hard to control these parameters with respect to various possible variations in industrial production. In this connection, above-mentioned Japanese Examined Patent Application Publication No. 03-54940 describes a tubular reactor as an example of reactors to be used but fails to teach a concrete example and concrete advantages of the tubular reactor.